Such implantable leads are typically, in particular, stimulation electrode leads (at times also referred to in short as “electrodes”) of cardiac pacemakers or shock electrode leads of implantable defibrillators, but they may also be catheters having an elongated conductive structure.
Medical implants, such as, for example, the pacemakers and defibrillators mentioned above, frequently have an electrical connection to the interior of the patient's body. Such a connection is used to measure electrical signals and/or to stimulate body cells. This connection is often configured as an oblong electrode. At present, electrical signals are transmitted between the implant and the electrode contacts (e.g., tip, rings, HV shock coils, sensors or the like) by way of materials offering good electrical conductivity.
When a system comprising an implant and an electrode is exposed to strong interference fields (e.g., EMI, MRI, etc.), undesirable error behavior may occur, especially the heating of parts of the system or electrical malfunctions (such as, for example, resets). Heating may result in damage to body tissue or organs when the heated parts have direct contact with the tissue. This is notably the case with the electrode tip.
The cause of the undesirable error behavior is the interaction of the field with the oblong lead structure of the electrode. The electrode acts as an antenna and receives energy from the surrounding fields. As the lead is used for treatment, the antenna can give off this energy distally by way of the electrode contacts (e.g., tip, ring, etc.) to the tissue, or proximally to the implant.
The same problems also occur with other oblong conductive structures, the proximal end of which is not necessarily connected to an implant (such as, for example, with catheters, temporary electrodes, and the like).
Shielded electrodes are known. In the prior art shielded electrodes, the shield of the electrode primarily acts against electrical fields that can be coupled from the outside. In addition, these shields are only effective and long-term stable when they have an appropriate shield strength. Consequently, a compromise must be found between enlarging the electrode diameter—with the attendant effects on costs and manageability of the electrode—and losses in the shielding effect.
In order to avoid interference due alternating magnetic fields, especially in magnetic resonance imaging (MRI) scanners, and more specifically to limit heating of the electrode tip in such fields, U.S. Publication No. 2008/0243218 proposes the provision of a protective conductor in an electrode lead which alternately turns back and forth in the longitudinal direction. This design, referred to as the billabong principle, also utilizes mutual inductances to reduce induced currents. However, in this case, an enlargement of the electrode diameter is also to be expected given the three-layer coil winding. In addition, the conductivity of the electrode will be lower.
From Ladd M., Quick H.: Reduction of resonant RF heating in intravascular catheters using coaxial chokes, Magnetic Resonance in Medicine, 2000, precautions are known against the heating, due to RF resonances, of intravascular catheters in the form of choke coils, referred to simply as chokes. Such chokes are located on the outer casing of the electrode and act against surface currents. However, this solution does not lower any currents that couple to the inside coil. In addition, an enlargement of the electrode diameter is to be expected, entailing the consequences described above.
The present invention is directed toward overcoming one or more of the above-mentioned problems
It is an object of the invention to provide an improved implantable lead of the type mentioned above, which exhibits improved properties in strong outside alternating magnetic fields and which has a simple design and is therefore inexpensive to implement.
This object is achieved by an implantable lead having the characteristics of the independent claim(s). Advantageous refinements of the inventive concept are the subject matter of the dependent claims.